The top engineer at Google 's DoubleClick unit is changing jobs as a new executive in charge of the business accelerates its push into mobile advertising.

Joerg Heilig, who had been vice president of engineering for display and video, recently left that job. In an email, Heilig said he will be moving to a different, but not yet announced, role at Google. He said the timing of the move was “influenced a bit by the recent changes in Ads management, but mostly by my desire to work on a very different engineering area.”

Heilig’s departure comes a few months after Sridhar Ramaswamy took full control of Google’s ads and commerce division. Ramaswamy had been sharing responsibility for the group with Susan Wojcicki until Wojcicki took the top job at YouTube in February.

One challenge facing DoubleClick has been catching up with rivals in mobile-advertising technology. Its technology was originally designed to support advertising on desktop computers. As smartphone usage takes off, publishing executives say rival products designed for mobile devices have technological advantages.

Consider MoPub, which was bought by Twitter last year. It is a leader in technology for so-called native advertising on smartphones, where ads are made to look like other content, like a promoted tweet on Twitter or a sponsored post on Facebook. Mobile app makers say native ads are bigger moneymakers on phones than banner ads. Customers have complained to DoubleClick that it doesn’t offer as much support for native ads on mobile devices.

Still, many customers are loath to move away from DoubleClick given how deeply they rely on its technology. DoubleClick includes a collection of advertising products that among other things provide web publishers and advertisers with the technology necessary to buy and sell online ads.